ci: separate whitespace check script

ci: separate whitespace check script

The `check-whitespace` CI job is only available as a GitHub action. To
help enable this job with other CI providers, first separate the logic
performing the whitespace check into its own script. In subsequent
commits, this script is further generalized allowing its reuse.

Helped-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>